Science‘s breakthrough of 2010: A visible quantum device

Science magazine's biggest scientific and technological breakthroughs of the …

At what point does a mechanical system—governed by classical dynamics—become a quantum system, or vice-versa? In 1920, Neils Bohr argued that quantum systems became describable with classical mechanics when the correspondence (or classical) limit had been reached. He described this as occurring "when the quantum numbers describing the system are large." The exact interpretation of "large" has been left to the reader as a 90-year-old take-home exercise.

The broad implications of Bohr's correspondence principle is that (as Bohr himself argued) one cannot derive classical mechanics from quantum mechanics; classical systems in the every-day world will be governed by classical laws, and the very small quantum systems by quantum laws. There are examples of macroscopic systems that obey quantum mechanics—Bose-Einstein condensates for example—but they are inherently quantum systems. However, in March of this year, a research group at the University of California, Santa Barbara reported on the first ever classical system—a vibrating mechanical resonator—where the behavior could be described and manipulated through quantum mechanical means.

The canonical approach for coaxing a classical system into the quantum realm is to cool it down until it reaches its "ground state." However, even the definition of what happens at the ground state differs between classical and quantum mechanics. Classical dynamics says that all kinetic energy in the device will be zero and no motion will occur. Quantum mechanics, on the other hand, describes irremovable random uncertainties where the system will actually exist in some bizarre superposition of a state with no quanta of energy and one where it has a single quanta of energy.

As an example, a simple mechanical beam fixed at one end and vibrating at an audible frequency of around 1kHz at the other would need to be cooled to a temperature significantly less than 50nK to reach its ground state. The temperature at which such a system will reach its ground state will increase proportionally to the frequency at which it vibrates. A stiffer beam will have a higher frequency, resulting in a higher ground state temperature. But it will also have a much smaller amplitude, which will make detection more difficult.

In a paper published in Nature, the team described a device that used piezoelectric materials to create what they called a "microwave-frequency 'quantum drum.'" They constructed a beam-like device about 60 microns in length that consisted of a sandwich of aluminum nitride between two sheets of aluminum. The aluminum nitride would expand and contract depending on the presence of an electric field (the piezoelectric effect).

The size, shape, and material properties of the device give rise to an isolated mechanical vibration mode near 6GHz (hence the microwave-frequency part of the name). At this frequency, the macro scale classical device should enter its quantum ground state at a temperature below 0.1K.

In order to determine if this resonator truly reaches its quantum ground state, the team connected it to a Josephsonphase qubit device. According to the authors, this coupling will allow "completely quantum-coherent measurement, preserving the quantum states in the coupled system." If the device were coupled to a classical measurement system, then there would be rapid decoherence of the quantum states—you'd never be able to measure its quantum behavior.

The entire device was cooled to around 25mK, a temperature where both the qubit and mechanical device should theoretically be in their quantum ground states. Once cooled, the team used the qubit to probe the nature of the mechanical oscillator, and found that the number of phonons (a vibrational quasiparticle) in the relevant mechanical mode was very small. There was a 93-plus percent chance that the resonator is in a quantum ground state.

By controlling the qubit, the researchers were able to show that they could impart a single quanta of energy—a single phonon—to the mechanical resonator and observe the exchange take place in real time. They were able to control this to such a degree that the mechanical device existed in a superposition of the ground state and single quantum state, having it actually move at two different amounts simultaneously. On the flip side, they also showed that they could use a classical excitation (microwaves) to generate a coherent response in the mechanical resonator, which excited the qubit in a manner that agreed very well with theory.

This set of experiments provided strong evidence for the first time that "quantum mechanics applies to a mechanical object large enough to be seen with the naked eye." While this tiny device won't be available in stores anytime soon, it opens the doors to a whole new range of experiments that can probe the limits of quantum mechanics and ask questions about the nature of reality itself.

A number of groups are currently working to build even larger mechanical systems that are governed by the laws of quantum mechanics, and still others are looking at ways to put this simple device to use in a whole range of applications. However, the fact that this was the first time that physics was able to build a classical machine that existed in a quantum state that got the editors at Science magazine to name it the breakthrough of the year for 2010.

20 Reader Comments

QFT assumes a Lorentzian manifold which can scale to masses large enough to see their diminutive effects in otherwise "classical" systems. Bohr's correspondence principle is simply a by-product of an unwillingness to accept the quantum revolution wholesale.

Well, sure. But to be fair, Bohr correspondence was true for every single classical object known up until this one. That's pretty good, if you consider it took 48 years after his death to build an exception.

Also, dear readers, emphasis on the fact that this is the first macroscopic device with quantum behavior (aside from BECs as noted). That makes a pretty good case for the fact that Deepak Chopra and other alt-med/woo peddlers who sling around the word "quantum" are thorough quacks and ripoff artists.

Also, dear readers, emphasis on the fact that this is the first macroscopic device with quantum behavior (aside from BECs as noted). That makes a pretty good case for the fact that Deepak Chopra and other alt-med/woo peddlers who sling around the word "quantum" are thorough quacks and ripoff artists.

Having some actual physics-related knowledge of quantum mechanics, when I was coerced into watching that piece-of-dreck movie "The Secret", I started laughing the first time the so-called "quantum physicist" came on screen.

This article really told me that I don't know much about quantum mechanics.

I thought flourescent lights, the basis for computer chips working at all, and generally anything that relied on electrons being in different atomic shells, and was limited by uncertainty, were manifestations of quantum devices on a commercial scale. I figured that's why you can't make mono-layer computer chips work, because the uncertainty of an electron crossing a single atom barrier was high enough to make the results unreliable.

I don't doubt the story's accuracy, but now I know why I got a BC in quantum mechanics back when I took it 18 years ago.

This article really told me that I don't know much about quantum mechanics.

I thought flourescent lights, the basis for computer chips working at all, and generally anything that relied on electrons being in different atomic shells, and was limited by uncertainty, were manifestations of quantum devices on a commercial scale. I figured that's why you can't make mono-layer computer chips work, because the uncertainty of an electron crossing a single atom barrier was high enough to make the results unreliable.

I don't doubt the story's accuracy, but now I know why I got a BC in quantum mechanics back when I took it 18 years ago.

Commercial, yes. This artifact, though, which exhibits the behavior does so on a scale that would be visible to the naked eye if it weren't for the fact that it needs to be in a dark box to work. The junction where the quantum effects happen in the examples you give, despite their producing visible results, are not visible to the naked eye.

I thought flourescent lights, the basis for computer chips working at all, and generally anything that relied on electrons being in different atomic shells, and was limited by uncertainty, were manifestations of quantum devices on a commercial scale.

You are right, but those are inherently quantum systems, things that rely on quantum mechanics to work properly. This is a vibrating bar, much like a diving board. Quantum mechanics, in general, doesn't tell us shit about its behavior, it is a purely classical device. The twist here is that this purely mechanical device is being governed by and controlled by quantum effects and it is large enough to be seen with the naked eye (if you squint really hard ). That is what is so new and novel about this.

I have suspected for some time that QM equations, though incomplete, are describing reality and not probabilities (or parallel universes or etc). The experiment that was very interesting to me years ago were discrete and independent quantum vortices in super cold fluids. Everything really IS waves, it only appears as particles because of how they interact.

Also, dear readers, emphasis on the fact that this is the first macroscopic device with quantum behavior (aside from BECs as noted). That makes a pretty good case for the fact that Deepak Chopra and other alt-med/woo peddlers who sling around the word "quantum" are thorough quacks and ripoff artists.

Pffft. Obviously these physicists wanted the results they got, so their thinking it collapsed the wave-form to produce it. Hmmm, time to make a car payment. I'm going to go collapse the wave-form of some potential gold bars.

I have suspected for some time that QM equations, though incomplete, are describing reality and not probabilities (or parallel universes or etc). The experiment that was very interesting to me years ago were discrete and independent quantum vortices in super cold fluids. Everything really IS waves, it only appears as particles because of how they interact.

I have suspected for some time that QM equations, though incomplete, are describing reality and not probabilities (or parallel universes or etc). The experiment that was very interesting to me years ago were discrete and independent quantum vortices in super cold fluids. Everything really IS waves, it only appears as particles because of how they interact.

Congratulations! You've rediscovered string theory.

I know, and in 1997 no less. When I being taught QM string therory wasn't completely accepted.

Is it now? I hadn't heard for sure.

Can’t help but add, it also means Einstein was right. God doesn’t play dice.

I know, and in 1997 no less. When I being taught QM string therory wasn't completely accepted.

Is it now? I hadn't heard for sure.

Can’t help but add, it also means Einstein was right. God doesn’t play dice.

String Theory? "Completely accepted?" A-hahahahahahahahahahaha. Which one? There are so many. I am not saying that one of the various theories isn't on it's way to being close to right...I'm just saying that we are a very long way away from licking string theory yet...

I know, and in 1997 no less. When I being taught QM string therory wasn't completely accepted.

Is it now? I hadn't heard for sure.

Can’t help but add, it also means Einstein was right. God doesn’t play dice.

String Theory? "Completely accepted?" A-hahahahahahahahahahaha. Which one? There are so many. I am not saying that one of the various theories isn't on it's way to being close to right...I'm just saying that we are a very long way away from licking string theory yet...

I was under the impression that string theory had been more or less abandoned. It seemed to me like it had hit something of a dead end, and further attempts to keep it relevant merely crossed the threshold into silly.

I was under the impression that string theory had been more or less abandoned. It seemed to me like it had hit something of a dead end, and further attempts to keep it relevant merely crossed the threshold into silly.

Oh...that depends entirely on which scientist you talk to. Short answer: no, it has not been abandoned. Not even close; many very reputable physicists continue to work at various aspects of the different theories all the time. That said, there are plenty of very learned physicists who call "hokum" on the whole thing. Generally, it is a subject of much debate and professional shenanigans. It is however very far from “dead.”

others are looking at ways to put this simple device to use in a whole range of applications.

What sorts of applications?

I swear I highlighted a list of things they (either the Science editors, or original paper's authors) mentioned, but damn if I can find it now. The one thing I recall was being able to use a similar device as a super-sensitive force detection method (sort of like turning AFM up a few notches), the Science recap mentions how the LIGO will use similar principles, but it isn't fully online yet and it wasn't what i was thinking of. The other potentially ground breaking experiments would come in seeing how and when quantum mechanics gives way to our observations of the world around us, i.e. can things be in two places at once, things like that...

I'll re-read the paper when I get a minute to see if I can't find that list again.

others are looking at ways to put this simple device to use in a whole range of applications.

What sorts of applications?

It see it as a beautiful sensor. Imagine the ability to harness natural vibrations or EM disturbances as a source of a truly random generator? Or something so sensitive that it can detect a magnetic fluctuation that is essentially a single quanta of energy.

Alternately, it seems like a neat way to CAUSE unbelievably precise electromagnetic changes. How much use any of that is I guess depends on how much use such precise tools are.

Matt Ford / Matt is a contributing writer at Ars Technica, focusing on physics, astronomy, chemistry, mathematics, and engineering. When he's not writing, he works on realtime models of large-scale engineering systems.